This photograph shows the microstructure of the high-thermal-conductivity aluminum diamond material used for heat sinks and in packages for high-power RF/microwave devices, such as GaN transistors. (Photo courtesy of Nano Materials International Corp.)

Handling The Heat In RF/MW Circuits

Thermal management of an RF/microwave component, circuit, or system is simply a matter of removing heat from sensitive areas of a design that can suffer damage or performance degradation from the heat. Of course, providing the right mix of thermally conductive materials to extract heat from an active source (such as a power transistor) or a thermal pathway (like a transmission line or circuit trace) may not always be so simple. For some designs, the addition of a component that may improve thermal management—e.g., a heat sink to an amplifier—may also thwart efforts at making the design as small as possible. But any attempt to understand the flow of heat through an electronic design can help improve the performance and reliability of that design. For most electronic components, circuits, and systems, maintaining a design at a lower operating temperature usually translates into improved performance and reliability.

The flow of heat through a high-frequency circuit can involve various input and output connectors and/or waveguide components. Most microwave components and systems, however, are built upon printed-circuit-board (PCB) materials and rely heavily on them for thermal management. Heat that is applied to or generated within a PCB material must flow away from the PCB materials and its active devices, then be dispersed in the equipment packaging, heat sinks, and ambient air. The choice of PCB material is therefore a key step in the thermal management of an RF/microwave high-power circuit or system.

Ideally, a PCB material can handle energy with a minimum amount of loss, with energy from on-circuit devices (such as transistors) or an external source (such as an amplifier from another circuit) transferred without generating undue heat. A circuit with a high amount of energy loss will transform some of the energy into heat, and that heat must be effectively dissipated to ensure the reliability of the circuit. An RF/microwave PCB is formed with dielectric materials and conductive metals, such as copper, to transfer high-frequency signals with minimal loss and distortion.

Because a PCB material will expand and contract with changing temperatures (caused by the heating effects of lost energy), the material components of a PCB are usually carefully selected. They usually have closely matched coefficient of thermal expansion (CTE) so that, for example, a PCB’s dielectric material and copper conductors will expand at the same rate (usually about 17 ppm/ºC) when power is applied or generated within a circuit on the PCB. Ideally, a PCB material has been engineered with dielectric and conductor that are closely matched in the three dimensions (x, y, and z or width, length, and thickness) of the material to minimize possible stress that can occur at joints between the conductors and the dielectric materials as they expand and contract.

The way a circuit is designed can also contribute to its thermal management. One example is through practical application of plated through holes (PTHs) to dissipate heat from an active device. Multiple PTHs can provide thermal paths from an active heat source—such as a power transistor—through a circuit’s dielectric layer or layers to a metal ground plane, dissipating the heat produced by the active device.

Manufacturers of RF/microwave integrated circuits (ICs) in surface-mount housings typically provide mounting instructions for their devices in terms of proper heat flow away from the component and through the PCB. The number of solder-filled PTHs, their diameters, and their density on the PCB are often specified for a particular active device to ensure that sufficient thermal flow is achieved through the PCB and to the ground plane, without also rendering a circuit board that is unfit for manufacturing. Some surface-mount IC manufacturers will go as far as providing measurements of the thermal resistance from the package junction to the PCB’s heat sink—of benefit for circuit designers desiring to incorporate the thermal models in their computer-aided-engineering (CAE) high-frequency simulation software.

A PCB material’s thermal conductivity, which is presented in watts of power per meter of material per degree Kelvin (W/mK), provides some indication of its effectiveness in dissipating heat, since it is a measure of the material’s capability to conduct heat. It can be used to compare the different rates of energy loss as heat through different materials. Quite simply, a PCB material with high value of thermal conductivity enables a circuit to operate at higher power levels with better heat flow away from active devices than a PCB material with lower value of thermal conductivity. In a PCB material, a conductor, such as copper, has very high value of thermal conductivity (about 400 W/mK) while the PCB’s dielectric material has very low value of thermal conductivity. In fact, the dielectric material serves as a thermal insulator.

However, the use of PTHs can help the flow and dissipation of heat from the top circuit layer through the dielectric layer to the bottom ground layer. In addition, different PCB material products can be compared by their composite thermal conductivity values, when comparing different materials for high-power applications in which a goal is to minimize operating temperature.

Controlling the temperature of a high-frequency circuit can have a direct impact on circuit performance since the relative dielectric constant of a PCB varies as a function of temperature. This quality is defined by a material parameter known as the thermal coefficient of dielectric constant, usually defined in units of ppm/ºC. Because changes in a PCB material’s dielectric constant result in changes in an RF/microwave circuit’s impedance (which is typically maintained at 50 Ω), it is critical to minimize temperature effects at high frequencies (and high signal power levels) to minimize signal reflections at mismatched impedances that can lead to amplitude losses and phase distortions. Tightly controlled thermal coefficient of dielectric constant and CTE characteristics are signs that a PCB material will deliver high levels of performance (with minimal swings in temperature) when handling high power levels.

Of course, in some extremely high-power applications, such as radar and electronic-warfare (EW) systems, designers may be facing the integration and thermal management of a high-power vacuum electron device such as a traveling-wave tube (TWT) or magnetron (see "Vacuum Devices Drive High Power"). As described in an application note from Communications & Power Industries, “Recommendations for Cooling High-Power Microwave Devices,” (publication AEB-31) multiple water baths are often necessary to safely transfer the heat produced by these devices away from the devices themselves and critical components within their systems.

Thermal management should be a high-level priority for any high-frequency design intended for high long-term reliability, especially if that design must operate at higher power levels. A wide range of PCB material products are currently available, with a wide range of performance parameters including low loss and high thermal conductivity. A PCB material’s MOT, while not the ultimate guideline for selecting a circuit material for high-power applications, can be used as a parameter for comparison among different PCB products for a potential application.

In some cases, newer RF power semiconductor devices, such as gallium-nitride-on-silicon-carbide (GaN-on-SiC) power transistors, are challenging the best designers of “thermally responsible” circuits with their extremely high power densities. In many cases, materials with higher thermal conductivities are being sought in place of or in addition to traditional PCB materials as a means of channeling heat away from these high-power-density semiconductor devices. The two-pronged challenge is in finding a heat spreader material that has a CTE close in value to the high-power semiconductor material, such as GaN or SiC, but also with high thermal conductivity.

After considerable research, work is currently being done on aluminum diamond metal-matrix-composite (MMC) materials (see figure) with extremely high thermal conductivity (500 W/mK or more) for efficient withdrawal of heat from GaN and other high-power semiconductors in high-frequency circuits. These aluminum diamond MMC materials have shown a great deal of promise in their capabilities of meeting this two-pronged challenge for reliable thermal management of high-power RF/microwave devices—either when used as heat sinks or as base materials in semiconductor packages.